Without math skills, who gets the best deal?

The Department for Education and Skills says 14.9m people lack numeracy skills expected of an 11-year-old child.

UK's Skills Minister Phil Hope says

...millions struggle to do the kind of basic sums that are so essential to our lives at work and at home

Is it mathematics skills per se, or are we talking about mental arithmetic skills? Personally, I think mental arithmetic is still important, not only for day to day shopping calculations ("15% off", and the like) but also for estimating any mathematics problem before you start it. For a lot of students, getting some sort of answer seems to be the main goal. Once they have an answer, they move on. Incredibly, a lot of them don't even check whether it was right or wrong if the answers are provided. And what I find is that many students don't even estimate the answer before they start and so they have no idea whether their final answer is reasonable. Estimation requires a reasonable level of mental arithmetic skills.

I found this interesting in the BBC article:

...campaign staff will be handing out percentage calculators and encouraging shoppers to test themselves to see if they could improve their skills.

Hmm - will handing out calculators help shoppers improve their mental arithmetic?